Method for passivating tinplate

ABSTRACT

The invention relates to a method for passivating the surface of a tinned steel strip which is moved through a coating system at a strip speed of at least 200 m/min. After the steel strip is tinned, the surface is anodically oxidized, and a liquid solution of a chromium-free aftertreatment agent is applied onto the oxide layer. The invention further relates to a tinned steel strip or sheet with a support layer made of a fine or ultrafine sheet of steel, a tin layer which is deposited thereon, and a surface layer of a chromium-free aftertreatment agent. An oxide layer is formed between the tin layer and the surface layer of the aftertreatment agent, said oxide layer substantially consisting of tetravalent tin oxide (SnO 2 ).

The invention pertains to a method for passivating the surface of tinnedsteel strips, which are moving at a strip speed through a coatingsystem, by anodic oxidation and post-treatment of the tinned steel stripwith a chromium-free post-treatment agent. The invention furtherconcerns the use of agents with complexly bound transition metals ororganic additives, which are normally used for dispersion or improvementof wetting, as chromium-free post-treatment agents in the production oftinned steel strips. A further object of the invention is a tinned steelstrip or sheet with a support layer made of a fine or ultrafine sheet ofsteel, a tin layer deposited thereon, and a surface layer of achromium-free post-treatment agent.

Tinplate is a versatile, economical and ecologically advantageousmaterial, which is used first of all in the packaging industry. Tinplateis a cold-rolled steel sheet with a thickness up to 0.5 millimeters,which is coated with a thin tin layer in order to protect the steelsheet from corrosion. The tin layer is deposited electrolytically, forexample, on the sheet.

In the production of tinned steel sheet, especially in electrolyticstrip tinning systems, the coated steel sheet is chemically orelectrochemically passivated and then coated with lubricant in order tomake the tinned steel sheet resistant to oxidation and in order toreduce the coefficient of friction so that the tinned steel sheet can bemore readily worked in the subsequent processing, for example in themanufacture of tin packaging for food. Solutions containing Cr⁶⁺ areusually used in the passivation. The use of chromium-containing agentsin the treatment of metal products is environmentally harmful andtherefore should be regulated by law, especially in the case of metalproducts that are intended for the food industry.

PRIOR ART

For this reason Cr⁶⁺-free passivation solutions were proposed in theprior art. For example, DE4205819A discloses a water-free compositionfor production of corrosion protection layers on metal surfaces, whichis based on silane compounds and compounds of tetravalent titanium orzirconium.

EP1002143A discloses a method for alkali passivation of zinc plated andzinc alloy plated steel surfaces and of aluminum and its alloys in stripcoating systems. Said passivation with an aqueous solution results in acorrosion protection layer that can serve as a base for subsequentcoating with enamel.

For the most part, these publications concern the passivation ofaluminum-containing and zinc-containing surfaces of hot-dip galvanizedfine sheet material and other hot-dip galvanized steel parts for use inthe automobile industry. On the other hand, there is not yet a methodthat can be used in practice for chromate-free passivation of tinplatethat affords satisfactory results.

EP1270764A discloses a surface-treated tinplate comprising an alloylayer on a surface of a steel sheet, a tin layer that has been depositedon the alloy layer so that the alloy layer is exposed over a surfacearea of 3.0% or more and a film comprising P and Si with a film weightof 0.5 to 100 mg/m² or 250 mg/m², is deposited on the exposed alloylayer and the tin layer.

Chromium-free post-treatment agents for tinplate are described in U.S.2009/0155621 A1 and the publications cited therein.

DISADVANTAGES OF THE PRIOR ART

When coated steel sheet is used to make food containers (tin cans),different requirements on the resistance to oxidation and thus thepassivation of the coated metal surface are imposed by comparison withthe requirements on said materials when intended for use in theautomobile industry. In particular, the passivation in this case has toprevent too strong a growth of the tin oxide layer during the storage ofthe coated steel sheet or the food container made therefrom until it iscoated with a protective enamel and furthermore until the preserved foodis consumed. In addition, the passivation should prevent discolorationsof the coated metal surface. Such discolorations arise, for example,when tin cans that contain sulfur-containing substances are sterilized,since the sulfur reacts with the tin in the coated steel surface if itis not sufficiently passivated. Because of the matte discoloration(marbling) or gold discoloration of the surface of the packaging, theconsumer may get the idea that the contents are tainted. Enamel adhesionproblems may also arise due to reaction with sulfur and these can beavoided by passivation of the coated steel sheet. The passivationmoreover must guarantee the resistance of the enameled metal container,after being filled with foods, to the acids contained in the foods, suchas mercaptoaminocarboxylic acid anions, for example cysteine andmethionine. If passivation is not sufficient, such acid anions in thecan contents can give rise to delamination of the inner coating of thecontainer.

In the traditional production of tinplate in strip tinning systems, thecold rolled fine or ultrafine sheet after tinning is first fused byheating to temperatures above the melting point of tin and then quenchedin a water bath. This is followed by passivation by treating the tinnedsteel sheet with a chromate solution and finally rinsing it withdemineralized water and drying it under heat. This is followed byelectrostatic lubrication with dioctyl sebacate (DOS) or acetyl tributylcitrate (ATBC). The chromate adsorbed on the surface is reduced to Cr³⁺by a reaction with the ═Sn═O and ═Sn—OH groups on the tin surface andalso in some cases to chromium metal in the case of electrochemicalcathodic passivation. The Cr³⁺ precipitates as Cr³⁺ hydroxide. Afterrinsing and drying the tinplate surface, the passivation layer no longercontains any Cr⁶⁺ ions.

PROBLEM OF THE INVENTION

Based on this, the invention has the problem of making available amethod for chromium-free passivation of the surface of steel strips thathave been coated with a tin coating, which are moving at a strip speedthrough a coating system, said method enabling efficient passivation ofthe steel strip surface even at high strip speeds. Moreover, at the sametime improved enamel coat adhesion and resistance to acids in foods,especially to amino acids that contain sulfur, should be achieved.

SOLUTION OF THE PROBLEM OF THE INVENTION

These problems are solved with a method that has the features of claim 1and with a tinned steel sheet or strip that has the features of claim17. Preferred embodiments are defined in the dependent claims.

In the method in accordance with the invention, after the fusion andquenching of the tinned steel strip and before the treatment with achromium-free post-treatment agent, the conduction of an anodicoxidation of the tin surface is provided. The invention proceeds fromthe knowledge that passivation of the tinned surface of the steel stripby post-treatment with a chromium-free post-treatment agent is not byitself sufficient to protect the surface completely and permanentlyagainst corrosion and discolorations (marbling). In particular, it hasbeen shown that chromium-free post-treatment agents do not protect thetinned steel strip surface against a reaction with sulfur everywhere. Ithas now been surprisingly established that the resistance of the tinnedsteel strip surface to corrosion and reaction with sulfur can beconsiderably increased if a deactivation of the tinned steel stripsurface by an anodic oxidation takes place first before thepost-treatment with a chromium-free post-treatment agent. An oxide layerwith a layer thickness in the nm range is produced on the tinned steelstrip surface by the anodic oxidation. The oxide layer is substantiallya layer of tetravalent tin oxide (SnO₂), which is considerably moreinert than divalent tin oxide (SnO). If now a thin surface layer of achromium-free post-treatment agent is deposited on said oxide layer, thesurface of the tinned steel strip will become completely and effectivelyprotected against corrosion and against a reaction with sulfur.

The method in accordance with the invention will be described below inmore detail by means of an embodiment example:

Step 1

In the first step of the method in accordance with the invention, anelectrochemical tinning of a cold rolled steel strip (fine or ultrafinesheet) takes place in a strip tinning system. The steel strip is movedthrough an electrolysis bath at a strip speed that usually lies in therange of 200 m/min to 750 m/min and is electrolytically coated with tin.Then the steel strip is heated to temperatures above the melting pointof tin (232° C.) conductively or inductively (or even conductively andinductively) in order to fuse the tin coating onto it. Then the movedstrip is quenched in a water bath. The tinned steel strip arrives withthis fresh surface at the second step of the method, namely anodicoxidation of the tin surface.

Step 2

In the second step of the method in accordance with the invention, ananodic oxidation takes place in a soda solution, i.e., in a sodiumcarbonate solution. The tinned steel strip is moved along at the stripspeed and is connected as the anode in the soda electrolyte bath. Anaqueous soda solution serves as the electrolyte. The concentration ofsodium carbonate in the soda solution is preferably 1 wt % to 10 wt %,particularly 2 wt % to 8 wt %, preferably 3 wt % to 7 wt %, above all 4wt % to 6 wt %, especially about 5 wt %.

The device for electrolytic anodic oxidation expediently comprises anelectrolysis immersion bath with a vertical tank. Near the bottom in thevertical tank is a reversing roller over which the tinned steel stripreverses travel. The vertical tank is filled with the electrolyte. Apotential is applied between the tinned steel strip and the counterelectrode (for example a steel cathode) in the vertical tank. The chargeQ is expediently between 0.2 C and 2 C, preferably between 0.2 C and 0.6C at a current density of 1-3 A/dm².

The anodization time corresponds to the residence time of the tinnedsteel strip in the electrochemical oxidation bath (electrolyte bath).This is determined by the length of the electrolyte bath or its filllevel and the anode length and the strip speed and for typical stripspeeds it expediently lies in the range of 0.1 s to 1 s, especiallybetween 0.1 s and 0.7 s, preferably in the range of 0.15 s to 0.5 s, andideally around 0.2 s. The anodization time, in dependence on the stripspeed, can be set via the fill level to values that are suitable inaccordance with the invention.

The spacing between the steel strip and the counter electrode in theelectrolyte bath is set in dependence on the system. It lies, forexample, in the range of 3 to 15 cm, preferably in the range of 5 to 10cm, and especially around 10 cm.

The temperature of the electrochemical oxidation bath preferably lies inthe range of 30 to 60° C., more preferably in the range of 35 to 50° C.,and especially around 45° C.

The current density is set in the range of 1.0 to 3 A/dm², preferably1.3 to 2.8 A/dm², more preferably 2 to 2.6 A/dm², especially around 2.4A/dm². The total charge in this case varies between 0.2 C and 0.6 C andis preferably, for example, 0.48 C. The corresponding charge densities(with respect to the area of the strip being treated) lie in the rangeof 0.2 C/dm² to 0.6 C/dm².

Step 3

In the third step of the method, there takes place a thorough rinsing ofthe tinned and oxidized steel strip with distilled or fullydemineralized water and a subsequent drying, which can take place, forexample, with hot air. However, other drying means are also suitable forthis, such as drying with water-absorbing solvents followed by dryingwith a cold or hot air blower, hot air being preferred, drying withconvection air-free drying systems like IR radiators, inductive heating,or resistance heating, or drying only with a cold or hot air blower,preferably a hot air blower.

Step 4

In the fourth step of the method, a coating of the tinned and oxidizedsteel strip surface with a post-treatment agent takes place.

For this, a solution of the post-treatment agent, preferably a solutionin water or an organic solvent or a ready to use preparation of thepost-treatment agent, is sprayed onto the steel strip, which is movingat the strip speed. 1.5 to 10% aqueous solutions of the post-treatmentagent proved to be expedient. Preferably the solution of thepost-treatment agent is then stripped off by squeeze rollers and dried.A thin film of the post-treatment agent still remains on the surface ofthe coated metal strip after the squeezing and drying, where the weightof said thin film as a rule is between 2 and 30 mg/m².

The post-treatment agent is sprayed, for example via tubes that arearranged at a distance from the coated metal strip surface andtransverse to the direction of strip travel and have holes or nozzlesthrough which the post-treatment agent reaches the coated steel stripsurface. Preferably at least one tube with such holes is arranged oneach side of the steel strip in order to splatter or spray both sides ofthe metal strip with the post-treatment agent. Preferably, the distancebetween the tubes and the tinned and anodically oxidized steel strip isadjusted and the position of the holes or nozzles with respect to thedirection of travel of the steel strip selected so that the exitingliquid treatment agent strikes the steel strip perpendicular to itssurface or at least within an angle range of ±45°, preferably within anangle range of ±15° about the normal (perpendicular) to the surface ofthe steel strip.

As an alternative to spraying the post-treatment agent onto the surfaceof the tinned steel strip, an application in an immersion process isalso possible.

To strip off the sprayed treatment agent, expediently two squeeze rollerpairs are arranged after the tubes in the direction of strip travel. Thespacing of the first squeeze roller pair from the tubes in the directionof strip travel is, for instance, between 20 and 100 cm. After thestripping, only a few molecular layers of the post-treatment solutionremains on the tinned steel strip surface, possibly only onemonomolecular layer of solution.

The solution stripped off by the squeeze rollers is collected in asupply tank, from which the excess post-treatment solution is optionallysent by pump to a preparation step and reused.

The agents defined below can be used as post-treatment agents. Theapplication of a Ti/Zr-containing post-treatment agent is described asrepresentative of this. The substance obtainable from Henkel KGaA underthe trade name Granodine® 1456, for example, is used as a Ti/Zrcontaining post-treatment agent. It is applied to the tinned andoxidized steel strip surface as a solution with a dry layer weight inthe range of 0.5 to 2 mg Ti/m², preferably 0.8 to 1.5 mg Ti/m²,especially around 1 mg Ti/m².

Step 5

As the fifth step, a drying step again takes place, where the dryingtemperature (strip temperature) lies in the range of 30 to 95° C. andpreferably between 35 and 60° C. The drying time is matched to the stripspeed. The drying devices indicated in step 3 can be used in this case.

The described method yields a tinned steel sheet with a layer structurecomposed as follows: at the bottom there is, as support, a cold rolledsteel sheet, in particular a fine or ultrafine sheet with a thickness of0.5 mm to 3 mm (fine sheet) or less than 0.5 mm (ultrafine sheet). Thetin layer, for example electrolytically deposited, follows as the nextlayer on the steel sheet. The amount of tin as a rule is 0.1 to 11.2g/m², but in individual cases it can even be less than 0.1 g/m² or morethan 11.2 g/m². Any alloy layers of support material and tin are ignoredhere as special intermediate layers. On the tin layer there then followsthe oxide layer produced by anodic oxidation with a thickness of a fewnm, which substantially consists of tetravalent tin oxide. The thicknessof the tin oxide layer varies roughly in the range of 2 to 10 nm. Thepost-treatment agent layer, which was deposited on the thin oxide layerby the post-treatment and subsequent drying, follows on the oxide layeras the surface layer. The weight of the post-treatment agent in the drystate (dry film weight) is expediently between 2 and 30 mg/m².

The sheet produced and post-treated in accordance with the inventionwill either be cut into sheet panels or rolled up as a coil. The enduser, for example the maker of tin cans, will as a rule then provide thesheet with an enamel layer, for example with a protective can enamel oran epoxide resin enamel. By comparison with the known tinplatematerials, the enameled sheet will then be processed by deep drawinginto molded objects, for example tin cans. The layer structure producedthrough the treatment in accordance with the invention contributes to animproved enamel adhesion and reduced flaking of the enamel, for examplebecause of the presence of mercapto group-containing amino acids.

The substances that are suitable for use in the method in accordancewith the invention for post-treatment of the surface of anodicallyoxidized tinplate must have properties such that they can adhere to thetin surface and at the same time allow wetting of the enamel layer thatis to be applied in the subsequent enameling of the tin surface. Thebonding between the functional group on the anodically oxidized tinsurface and the functional groups of the enamel surface with theiradhesion mediating molecules must be strong enough after drying theenamel film that they are [sic; it is] not disrupted in the cysteinetest (sterilization of the enameled tinplate for 90 min at 121° C. in asolution of 3.65 g/L KH₂PO₄ with 7.22 g/L Na₂HPO₄·2H₂O and 1 g/Lcysteine). In addition, the substances used for post-treatment should bechromium free and usable without the addition of organic solvents, or besoluble in distilled water, since the use of dissolving aids produceshigh concentrations of solvent in the exhaust air and thus wouldnecessitate expensive purification systems to remove the solvent.

Substances that in practice are preferably used as additives for betterdispersion of pigments in enamels or to improve the wetting and/oradhesion of enamels to metal surfaces, for example, prove to beespecially suitable substances as post-treatment agents. Such agents arechosen from copolymers of acrylates, polymethylsiloxanes with polyetherside chains, acid polyethers, and polymers with heterocyclic groups.However, substances that are used as corrosion protection agents forsheet metal, for example in the automotive industry, for examplebonderizing or Parkerizing agents can also be advantageously used. Suchsubstances are chosen from acidic, aqueous, liquid compositions thatcontain complex metal-fluoride anions with di- to tetravalent cationsand polymer substances.

Besides copolymers of acrylates, the following substances proved to beespecially suitable as post-treatment agents for the method inaccordance with the invention:

-   -   a) an organically modified polysiloxane with refractive index of        1.456 to 1.466 per DIN 53491 and a density at 20° C. of        1.09-1.13 g/cm³ per DIN 51757 (commercially available under the        name “EFKA 3580”),    -   b1) a polymer with a chemical composition of an acid polyether        with a density of 1.20-1.30 g/cm3 per DIN 51757 and an acid        value of 270-310 mg KOH/g per DIN 53402 [EFKA 8512],    -   b2) a polyacrylate modified with fluorocarbon residues, which        has been neutralized with dimethylethanolamine, where an aqueous        solution containing 59 to 61 wt % active substances has a        density at 20° C. of 1.04 to 1.06 g/cm3 per DIN 51757, a        refractive index of 1.420 to 1.440 per DIN 53491, and an acid        value of 50-70 mg KOH/g per DIN 53402 [EFKA 3570],    -   b3) a modified polyacrylate that has, for a content of 38-42 wt        % active substance in water, a density of 1.02 to 1.06 g/cm3 per        DIN 51757 and an amine number of 22 to 28 mg KOH/g per DIN 16945        [EFKA 4560],    -   c) a polymer that contains:        -   i) 0-80 mol % of one or more polymers of the formula

-   -   -   where R₁, R₂, R₃, and R₄ can be the same or different and            represent H or an alkyl,        -   ii) 0-70 mol % of one or more monomers of the formula

-   -   -   in which R₅, R₆, and R₇ can be the same or different and            represent H or an alkyl, and R₈ represents an alkyl or a            substituted alkyl, and the alkyl group R₈ can be interrupted            by —O— groups,        -   iii) 5-50 mol % of one or more monomers containing a            heterocyclic group with at least one basic ring nitrogen            atom, or in which such a heterocyclic group is present after            a polymerization,        -   iv) 0-10 mol % of one or more monomers containing one or            more groups that are reactive for crosslinking or coupling,            and        -   v) 0-20 mol % of one or more monomers that do not fall into            the preceding groups i)-iv), where the amount of the monomer            of group i) together with monomers that contain an acrylate            group is at least 20 mol %, and organic salts thereof, or

    -   d) an acidic, aqueous, liquid composition that contains, besides        water and optional solvents, the following components:        -   (A) fluorometallate anion with titanium, zirconium, hathium,            silicon, aluminum, and/or boron as central atoms; and            optionally ionizable hydrogen atoms, and/or optionally one            or more oxygen atoms;        -   (B) one or more di- to tetravalent, especially divalent            and/or tetravalent cations of cobalt, magnesium, manganese,            zinc, nickel, tin, copper, zirconium, iron, and/or            strontium;        -   (C) phosphorus-containing inorganic oxyanions and/or            phosphonate anions, and        -   (D) one or more water-soluble and/or water-dispersible            organic polymers and/or polymer-forming resins.

Preferably, the substance (d) is an acidic, aqueous, liquid compositionthat contains the following components in addition to water:

-   -   (A) fluorometallate anions that each consist of: (i) at least        four fluorine atoms and (ii) at least 1 atom of a metallic        element that is chosen from the group consisting of titanium,        zirconium, hafnium, silicon, aluminum, and boron; and        optionally (iii) ionizable hydrogen atoms, and/or        optionally (iv) one or more oxygen atoms;    -   (B) an amount of one or more di- to tetravalent, especially        divalent and/or tetravalent cation(s) of cobalt, magnesium,        manganese, zinc, nickel, tin, copper, zirconium, iron, and/or        strontium, so that the ratio of the total number of cation        (cations) to the number of anions in the component (A) is at        least one 1:5, but no more than 3:1;    -   (C) phosphorus-containing inorganic oxyanions and/or phosphonate        anions, and    -   (D) one or more water-soluble and/or water-dispersible organic        polymers and/or polymer-forming resins, where the amount of        these components is such that the ratio of the solids content of        organic polymer and polymer-forming resin in the composition to        the solids content of the component (A) lies in the range of 1:2        to 3:1.

In particular, the substance d) is an acidic, aqueous, liquidcomposition that is substantially free of hexavalent chromium and/orferricyanide, for treatment of metal surfaces, which contains no morethan 1.0% and preferably no more than 0.0002% of both hexavalentchromium and ferricyanide and that otherwise contains the followingcomponents in addition to water:

-   -   (A) at least 0.010 M/kg [sic] fluorometallate anions, where said        anions respectively consist of: (i) at least four fluorine atoms        and (ii) at least 1 atom of a metallic element that is chosen        from the group that consists of titanium, zirconium, hafnium,        silicon, aluminum, and boron; and optionally (iii) ionizable        hydrogen atoms, and/or optionally (iv) one or more oxygen atoms;    -   (B) an amount of di- to tetravalent, especially one or more        divalent and/or tetravalent cation(s) of cobalt, magnesium,        manganese, zinc, nickel, tin, copper, zirconium, iron, and/or        strontium, so that the ratio of the total number of cation        (cations) to the number of anions in the component (A) is at        least 1:5, but no more than 3:1;    -   (C) at least 0.015 MP/kg [sic] and preferably at least 0.030        Mp/kg [sic] phosphorus-containing inorganic oxyanions and/or        phosphonate anions, and    -   (D) at least 0.10% and preferably at least 0.20% of a        water-soluble (or more water-soluble) and/or water-dispersible        (more dispersible in water) organic polymer (organic polymers)        and/or polymer forming resin (polymer forming resins), where the        amount of said component is such that the ratio of the solids        content of the organic polymer (organic polymers) and the        polymer forming resin (polymer forming resins) in the        composition to the solids content of the component (A) lies in        the range of 1:2 to 3:1.

Preferably, the component d) comprises dihydrogen hexafluorotitanate(2-) and organic polymers. Especially preferably is the componentGranodine 1456.

The polymer (D) can be a polymer compound that comprises a copolymermaterial, where at least a part of the copolymer has the structure

which is defined more precisely in claim 1 and the dependent claims ofthe European patent application with the Publication Number EP0319017A2.

Such a polymer can be prepared, for example, as follows:

A plastic flask is filled with 400 mL Propasol P (a propoxylated propanesolvent obtainable from Union Carbide Corp., Danbury, Conn.) and 160 gresin M (a polyvinyl phenol, obtainable from Maruzen Oil, MW=5000). Aslurry of 263.3 g N-methylglucamine in 400 [sic] deionized water isadded and the mixture is heated to 60-65° C. while stirring. Then 100.2mL 37% formaldehyde is added over one to one and a half hours. Themixture is then heated to 90° C. and held for 6 hours. After cooling,the mixture is diluted to 9.6 wt % solids with deionized water. The pHof the prepared solution is 9.1 and the solution comprises anN-methylglucamine derivative.

Also suitable are the polyphenol derivatives in accordance with claim 1and the dependent claims of the European patent application with thepublication number EP 0 319 016 A2:

Such a polymer can be prepared, for example, as follows:

80 g of a resin of the above formula with an average molecular weight ofabout 2400, in which R₁, R₁₃, R₁₄, and W₂ are H, R₂ is —CH₃, and Y₂ isan alkylamine residue or an alkylammonium residue (with reference to theFormula I given in EP 0 319 016 A2 and its definitions), are slowlydissolved in 160 mL Propasol P (a propoxylated propanol solventobtainable from Union Carbide Corp., Danbury, Conn.) in a 1 literplastic flask using a high speed mixer. The plastic flask is fitted witha paddle stirrer, a reflux condenser, and a nitrogen flush device. Then53.5 g 2-(methylamino)ethanol and 160 mL deionized water are added tothe resin solution. Careful heating to 60° C. is begun. When 60° C. isreached, the addition of 50 mL 37% formaldehyde in water is begun, andthis addition is continued over a time of 1 h. Another 25 mL Propasol Pare added, and the reaction mixture is held at 60° C. for 1.5 h. Thetemperature is raised to 80° C. and held there for 1.75 h. The reactionmixture is allowed to cool to room temperature, and 21.8 g 75% H₃PO₄followed by 960 mL deionized water are added. Optionally, an amineoxide, which does not require an additional neutralization step, isformed by adding 0.75 mol 30% H₂O₂ (85 g) to the reaction mixture. Thereaction mixture is stirred overnight and then diluted with 960 mLdeionized water. The result of this optional step is a water-solubleamine oxide resin, which does not require neutralization for waterstability.

Suitable post-treatment agents with complexly bound transition metalsare, for example, products of the Granodine® series, which are aqueoussolutions containing complexly bound transition metals (Ti, Mn, Zr) andoptionally a silane. Titanium- and/or zirconium-containingpost-treatment agents proved to be especially suitable, for exampleproducts of the Granodine® series (manufacturer: Henkel) or Gardobond®series (manufacturer: Chemetall), especially the products “Granodine®1456” and “Gardobond® X 4707.”These titanium- and zirconium-containingproducts are expediently metered as post-treatment agents for the methodin accordance with the invention so that after the squeezing out anddrying, a titanium weight (dry weight) of 0.5 m g/m² to 2 mg/m² andespecially about 1.0 mg/m² is present on the surface of the treatedtinplate. Typically for this, 1.5-10% aqueous solutions of saidpost-treatment agents are used, where the fraction of titanium in theaqueous solution is preferably between 0.2 and 1.2 g/L and especiallypreferably between 0.2 and 0.5 g/L.

Components (a) to (c) were developed by EFKA Chemicals or EFKA Additivesand are currently made available by BASF SE. The products are describedin detail in the publications U.S. Pat. No. 5,688,858A, EP0311157 A1(page 12, line 45 to page 13, line 36, and the examples), U.S. Pat. No.5,399,294A, EPO438836 A1 (column 10, line 42 to 57, and the examples),U.S. Pat. No. 5,882,393A, WO97/26984 A1 (page 20, lines 4 to 20, and theexamples), U.S.2004063828A, WO02/057004 A1 (page 11, lines 1 to 6, andthe examples), U.S.2004236007A, WO03/033603 Al (page 20, lines 1 to 23,and the examples), U.S.2009234062A, WO2004/045755 A2 (page 21, lines 2to 13, and the examples), U.S.2007293692A and WO2005/085261 A1 (page 14,lines 1 to 25, and the examples) especially in the independent claimslisted there.

The following products from BASF SE are preferred: EFKA 3570, EFKA 3580,EFKA 4560, and EFKA 8512. EFKA 4560 is especially preferred.

Acrylate-containing agents with N-heterocyclic groups proved to beespecially suitable for the post-treatment agents, for example EFKA4560. These agents correspond to the above defined components (c).

They can be prepared, for example, as follows:

2.84 g vinyltoluene, 4.55 g isobutyl methacrylate, 7.36 g ethylhexylacrylate, 5.20 g hydroxyethyl methacrylate, 1.80 g polyethylene glycolmonomethacrylate with a molecular weight of about 400, and 0.44 gdi-tert-butyl peroxide are dissolved in 9.86 g xylene and 4.93 gmethoxypropyl acetate in a reaction flask that is suitable forpolymerizations and that is equipped with a reflux condenser. Thepolymerization was carried out at the boiling point of the mixture withstirring and with feeding of an inert gas. At the end of thepolymerization, 9.79 g isophorone diisocyanate were dissolved in 16.58 gisobutyl acetate and 16.58 g methoxypropyl acetate, and the remainingfree NCO groups were then reacted with 3.60 g polyethylene glycolmonomethacrylate with a molecular weight of about 400 and 4.51 g1-(3-aminopropyl)imidazole.

The solids content was then adjusted to 40 wt % with butyl acetate.

Similarly, 3.54 g vinyltoluene, 5.69 g isobornyl methacrylate, 9.20 g2-ethylhexyl methacrylate, 7.15 g hydroxyethyl methacrylate, and 1.28 gdi-tert-butyl peroxide dissolved in 11.94 g xylene and 5.97 gmethoxypropyl acetate were polymerized.

Then 12.23 g isophorone diisocyanate dissolved in 20.36 g butyl acetateand 20.36 g methoxypropyl acetate were added. The remaining free NCOgroups were then reacted with 4.50 g polyethylene glycolmonomethacrylate with a molecular weight of about 400 and 3.78 g3-amino-1,2,4-triazole in 11.34 g N-methylpyrrolidone.

The solids content was then adjusted to 40 wt % with butyl acrylate.

In accordance with the above described method of the invention, thesesubstances are sprayed in undiluted form or as an aqueous solution ontothe tinned steel strip and optionally then squeezed off and dried. Dryfilm weights in the range of 2 to 15 mg/m² and preferably between 2 and10 mg/m² are suitable as application weights for, for example, thesubstance EFKA 4560.

With the post-treatment agent(s), the (further) zinc oxide buildup onthe coated metal strip surface during its storage before being coatedwith lubricant can be highly reduced. At the same time, with thetreatment of the anodically oxidized, tinned steel strip surface withsaid post-treatment agents in accordance with the invention, there is animprovement of enamel adhesion. The tinplate surfaces treated inaccordance with the invention proved to be very readily coatable withenamel.

As a result of the treatment of tinplate with an anodic oxidation andthe application of a chromium-free post-treatment agent in accordancewith the invention, it turned out that sulfur-containing substances,which are present in foods and derived, for example, from the aminoacids cysteine or methionine, are no longer able to pass by diffusionthrough the subsequently applied enamel coating layer and thepassivation in accordance with the invention to the tin surface and togive rise to unsightly marbling flaws there (yellow or browndiscolorations), which are substantially due to the formation of tinsulfide, and which in an extreme case can lead to deterioration of theadhesion of the enamel to the tinplate or to separation of the enamelcoating.

The tinplate samples treated in accordance with the invention in somecases have clearly lower sliding friction than untreated tinplate, andwith individual substances it is even possible to obtain lowercoefficients of friction than the traditional treatment of the tinplatewith DOS.

From a thorough examination of the results of comparative experiments,it can be concluded that, with the method in accordance with theinvention for passivation of the surface of tinplate by anodic oxidationand coating with the post-treatment agents used in accordance with theinvention, the best results with regard to resistance to oxidation andmarbling (sulfide blocking activity) and friction are obtained. Theresults are comparable with the quality of the passivation of tinplatewith chromium-containing post-treatment agents that is known from theprior art.

The substances EFKA 3580, EFKA 4560, EFKA 8512, and EFKA 3570 producevery good results with regard to the properties low coefficients offriction, resistance to oxidation (low tin oxide buildup), andresistance to marbling (sulfide blocking activity). In this regard EFKA4560 turned out to be the preferred post-treatment agent. With regard toresistance to marbling and enamel adhesion, the titanium-containingpost-treatment agents of the Granodine® series (especially Granodine®1456) and Gardobond® series (especially Gardobond® X 4707) show veryadvantageous results. However, other chromium-free post-treatmentagents, in some cases already known from the prior art, are alsosuitable for conducting the method in accordance with the invention. Themethod in accordance with the invention can be employed both in the caseof tinplate with a fused-on tin layer as well as in the case of atinplate in which the tin layer has not been fused on.

1. Method for passivating the surface of a tinned steel strip, which ismoving at a strip speed of at least 200 m/min through a coating system,wherein after tinning the steel strip the surface is first anodicallyoxidized in order to form an oxide layer, which substantially consistsof tetravalent tin oxide (SnO₂), and then a liquid solution of achromium-free post-treatment agent is applied to the oxide layer. 2.Method of claim 1, wherein the anodic oxidation is conducted with abasic electrolyte and at a current density of 1.0 A/dm² to 3 A/dm² bypassing the tinned steel strip through an electrolysis bath.
 3. Methodof claim 2, wherein the basic electrolyte is chosen from an alkali metalor alkaline earth metal hydroxide or carbonate, a basic alkali metalphosphate, and a basic organic alkali metal or alkaline earth metalsalt.
 4. Method of claim 2, where wherein the electrolyte containssodium carbonate.
 5. Method of claim 2, wherein the electrolysis bath isadjusted so that a charge of at least 0.2 C is achieved.
 6. Method ofclaim 1, wherein the post-treatment agent is chosen from copolymers ofacrylates, polymethylsiloxanes with polyether side chains, acidpolyethers, polymers with heterocyclic groups, and acidic, aqueous,liquid compositions that contain complex metal fluoride anions with di-to tetravalent cations and polymer substances.
 7. Method of claim 1,wherein the post-treatment agent contains titanium and/or zirconium. 8.Method claim 1, wherein after the anodic oxidation, an aqueous solutionof the post-treatment agent or a ready to use solution of thepost-treatment agent is applied by a passage through a bath containingthe post-treatment agent.
 9. Method claim 1, wherein the post-treatmentagent is sprayed through at least one tube, which is arranged at adistance from the coated metal strip surface and has at least one holeor nozzle through which the post-treatment agent is sprayed onto one oreach coated surface of the steel strip.
 10. Method claim 1, wherein thesolution of the post-treatment agent is applied to both sides of thesteel strip.
 11. Method of claim 9, wherein the post-treatment agent issprayed onto the tinned steel strip surface(s) in the form of liquidjets, where the liquid jets strike the surface in an angle range ofbetween +45° and −45° to the normal to the surface.
 12. Method of claim1, wherein the solution of the post-treatment agent, after being appliedto the tinned and oxidized surface of the steel strip, is squeezed offby means of squeeze rollers.
 13. Method of claim 12, wherein the tinnedsteel strip is dried after squeezing off the post-treatment agent. 14.Method as in claim 13, wherein after squeezing off the post-treatmentagent and drying, a thin film of the post-treatment agent with a weightbetween 2 mg/m² and 30 mg/m² is present on the tinned steel strip. 15.Method as in claim 13, wherein the post-treatment agent istitanium-containing and that after squeezing off the titanium-containingpost-treatment agent and after drying, a thin film of the post-treatmentagent with a titanium weight between 0.5 mg/m² and 2 mg/m² is present onthe surface of the tinned steel strip.
 16. Method of claim 1, whereinthe anodic oxidation of the tinned surface of the steel strip takesplace within an anodization time of 0.1 to 1.0 second and preferablywithin an anodization time between 0.2 and 0.7 second.
 17. Tinned steelstrip or sheet with a support layer of a fine or ultrafine sheet ofsteel, a tin layer deposited thereon, and a surface layer of achromium-free post-treatment agent, characterized in that between thetin layer and the surface layer of the post-treatment agent, an oxidelayer is formed, which substantially consists of tetravalent tin oxide(SnO₂).